Phase modulation schemes are very effective and are therefore widely used in communication systems. A simple example of a phase modulation scheme is quaternary phase shift keying (QPSK). FIG. 1 shows a constellation diagram that illustrates how QPSK maps two-bit digital data to one of four phase offsets. FIG. 2 shows a typical QPSK (or in-phase (I)/quadrature (Q)) modulator used to generate a phase-modulated signal. This technique relies on orthogonal signal vectors to realize the phase offsets—an inherently linear technique, since it depends solely on the matching of these orthogonal signals.
The I/Q modulator provides a straightforward approach to generating phase-modulated signals that is also suitable for more complex schemes such as wideband Code-Division Multiple Access (CDMA) and Orthogonal Frequency Division Multiplexing (OFDM) systems. It is also possible to generate the phase-modulated signals using a phase-locked loop (PLL). This approach offers reduced circuitry and lower power consumption and, as a result, finds widespread use in narrowband systems. Unfortunately, the flexibility of the voltage-controlled oscillator (VCO) within the PLL architecture is limited. This is a severe disadvantage in multi-mode systems. It would therefore be advantageous to have a flexible, multi-mode VCO for use by a phase modulator.